Add Content-Length header to HTTP response in FilesRouter (#2408)

src/Routers/FilesRouter.js

@@ -40,6 +40,7 @@ export class FilesRouter {
       res.status(200);
       var contentType = mime.lookup(filename);
       res.set('Content-Type', contentType);
+      res.set('Content-Length', data.length);
       res.end(data);
     }).catch((err) => {
       res.status(404);